Thermally Stable Blue-Light-Emitting Hybrid ... - ACS Publications

9624

Macromolecules 2008, 41, 9624-9636

Thermally Stable Blue-Light-Emitting Hybrid Organic-Inorganic Polymers Derived from Cyclotriphosphazene Jianwei Xu,*,† Cher Ling Toh,† Karen Lin Ke,† Jason Jiesheng Li,† Ching Mui Cho,† Xuehong Lu,‡ Ee Wah Tan,‡ and Chaobin He† Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 3 Research Link, Singapore 117602, and School of Materials Science and Engineering, Nanyang Technological UniVersity, Nanyang AVenue, Singapore 639798 ReceiVed July 11, 2008; ReVised Manuscript ReceiVed September 3, 2008

ABSTRACT: The synthesis and characterization of thermally stable cyclotriphosphazene-based hybrid organic-inorganic blue-light-emitting polymers were reported. Monomers 2,4-di(4-bromoaryl)-2,4,6,6-tetra(phenoxy)-cyclotriphosphazene (3 and 4) and 2,4-di(4-bromoaryl)-2,4,6,6-tetra(ethoxy)-cyclotriphosphazene (7 and 9) were prepared beginning from hexachlorocyclotriphosphazene through a two-step reaction with the corresponding sodium 4-bromo-aryloxides. Next, 3, 4, 7, or 9 readily underwent a Suzuki cross-coupling reaction with suitable 9,9-dialkylflourene-2,7-bis(trimethyleneborate)s to afford the corresponding polymers. The structures and properties of the polymers were investigated by thermogravimetric analysis, differential scanning calorimetry, X-ray diffraction, and UV-vis and fluorescence spectroscopy. All polymers emitted blue light with high quantum yields. Subsequently, the thermal effect on the optical properties of the polymers was studied, revealing that the introduction of particular hybrid organic-inorganic three-dimensional cyclotriphosphazene in the polymer system prevented from aggregation of the polymer chain, enhanced thermal stability, and thus suppressed the undesirable green emission. On the basis of these studies, light-emitting polymer-based diode devices were fabricated.

Introduction Poly(9,9-dialkylfluorene)s and conjugated fluorene alternating copolymers have been the subject of intense research efforts due to their blue light-emitting characteristics, high emission efficiencies, and structurally facile functionalization at the 9-position of fluorene monomers. There are increasing applications of these polymers in electronics and optoelectronics, particularly in the areas of polymer light-emitting diodes (PLED)1,2 and organic thin-film transistors (OTFT).3,4 However, poly(9,9-dialkylfluorene)s and fluorene alternating copolymers suffer from the undesirable appearance of a new broadband at long-wavelength around 530 nm during either device operation or thermal annealing, generally originating from the rearrangement of the polymer chain, and subsequent aggregate or excimer formation.5-10 It has also been hypothesized that this band is due to the formation of keto defects11-15 induced by the oxidative degradation of C-9 at the fluorene unit. The resulting long-wavelength emission can lead to the substantial loss of color integrity accompanied with a reduction of the overall quantum efficiency, subsequently deteriorating the device performance. Therefore, significant efforts have been made toward structural modification of fluorene-based monomers to reduce or even eliminate the green emission at wavelengths around 530 nm. The synthetic strategies employed to reduce the green emission focus primarily on structural modification of the 9-position of the fluorene unit. These methods include the introduction of bulky groups such as sterically hindered side chains,16 dendritic moieties,17 spiro-linked groups,18,19 and the replacement of the susceptible C-9 of fluorene by a silicon atom.20 Other effective methods to decrease the long-wavelength emission include the attachment of oligofluorene units into hyperbranched, star-like, or dendritic architectures21-23 and the * To whom correspondence should be addressed. E-mail: jw-xu@ imre.a-star.edu.sg. Telephone: (65)-6872-7543. Fax: (65)-6872-7528. † Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research). ‡ School of Materials Science and Engineering, Nanyang Technological University.

formation of fluorene alternating copolymers by incorporation of a comonomer with a variety of bulky side groups24-30 These modified poly(fluorene)s and fluorene alternating copolymers have displayed improved optical stabilities,19,22,31 and, in some cases, they have remarkably suppressed green emission.20,23 Alternatively, introduction of three-dimensional structure with an inorganic component in a luminescent polymer system provides a novel mechanism by which to control the unfavorable green emission of these polymers. The polymers with threedimensional structures tend to form amorphous phases, which inhibit the formation of detrimental π-aggregates that are, in part, responsible for the green emission. Furthermore, the presence of molecularly linked inorganic components in a polymer backbone can enhance thermal stability, decrease flammability, and increase oxidation resistance, thus minimizing the green emission of polymers. Cyclotriphosphazene, composed of alternating nitrogen and phosphorus atoms, is a commonly used building block with an inorganic (PdN)3 core, and it is extensively used as an interior core or pendent group to develop dendrimers or polymers, respectively. In most cases, cyclotriphosphazene-based materials are prepared form commercially available hexachlorocyclotriphosphazene ((PN)3Cl6), which can be readily modified with a variety of substituents Via nucleophilic substitution.32,33 The good reactivity of the P-Cl bond toward nucleophiles offers synthetic adaptability to introduce a large number of substituents with appropriate functionality, which can subsequently be transformed into desired synthetic precursors. Thus, it allows for the preparation of various types of molecules and polymers for varied applications, including flame retardants,34,35 biodegradable materials,36 liquid crystals,37-41 and luminescent materials.42 Herein, we report the synthesis of cyclolinear polymers with cyclotriphosphazene units and blue light-emitting fluorene groups in the polymer backbone. The introduction of cyclotriphosphazene moieties improves the oxidative stability and, most importantly, the thermal stability of optical properties of the resulting luminescent materials.

10.1021/ma801563s CCC: $40.75  2008 American Chemical Society Published on Web 10/23/2008

Macromolecules, Vol. 41, No. 24, 2008

Experimental Section Materials. Hexachlorocyclotriphosphazene was purified by recrystallization in ethyl acetate. Other commercially available reagents and solvents were used without further purification. Instrumentation. 1H, 13C Nuclear Magnetic Resonance (NMR) and 31P NMR spectra were recorded on a Bruker DRX 400-MHz NMR spectrometer in CDCl3 at room temperature. Spectrometer operating frequencies were 400.13 MHz (1H), 100.61 MHz (13C), and 161.98 MHz (31P). Tetramethyl silane (TMS) was used an internal standard for 1H and 13C NMR spectra, and 85% phosphoric acid was used an external standard for 31P NMR spectra. Fourier transform infrared (FT-IR) spectra were acquired on a Perkin-Elmer Spectrum 2000 FT-IR spectrometer. Electron ionization mass spectra (EIMS) and electrospray ionization (ESI) mass spectra were recorded using a Micromass 7034E and Finnigan TSQ 7000 triple stage quadrupole mass spectrometer, respectively. Elemental analysis was conducted on a Perkin-Elmer 240C elemental analyzer for C, H, and N determination. Thermal analysis was performed in a Perkin-Elmer thermogravimetirc analyzer (TGA 7) in nitrogen or in air at a heating rate of 20 °C min-1 and a TA Instruments Differential Scanning Calorimetry (DSC) 2920 at a heating rate and a cooling rate of 10 °C min-1 in nitrogen. Gel permeation chromatography (GPC) analyses were carried out on a Waters 2690 system using polystyrene as standards and THF as eluent. Wide angle X-ray diffraction experiments were conducted in a Bruker GADDS X-ray diffractometer using Cu KR 1.5418 Å radiation. UV-vis and fluorescence spectra were obtained using a Shimadzu UV3101PC UV-vis-NIR spectrophotometer and a Perkin-Elmer LS 50B luminescence spectrometer with a Xenon lamp as light source, respectively. The photoluminescence efficiency (φPL) of the polymers in THF solutions was measured by using quinine sulfate (1 × 10-5 M solution in 0.1 M H2SO4, assuming φPL of 0.55) as a standard. trans-/cis-2,4-Dichloro-2,4,6,6-tetra(phenoxy)cyclotriphosphazene (trans-/cis-2). Sodium phenoxide was prepared by directly adding phenol (22.71 g, 0.24 mol) in tetrahydrofuran to sodium hydride (9.70 g, 0.24 mol, 60% dispersion in mineral oil) (100 mL). Then the above solution was added dropwise to hexachlorocyclotriphosphazene (20.00 g, 0.057 mol) in tetrahydrofuran (75 mL) at room temperature. The resulting reaction mixture was allowed to reflux and stir for 12 h under nitrogen. After the reaction was complete, the solvent was then removed under reduced pressure. An appropriate amount of chloroform followed by water was added to dissolve the residue. The organic layer was washed with dilute hydrochloric acid and water. Finally the organic layer was dried with anhydrous magnesium sulfate and was removed under reduced pressure. The residue was chromatographed using chloroform/ hexane as eluent to afford the corresponding mixture of cis-2 and trans-2 as a viscous liquid. Yield: 55%; the ratio of cis-2 and trans-2 estimated from the integration of phosphorus in their 31P NMR spectra is 53%:47%. 31P NMR: δ 20.80 (d, 2JP-P ) 80.1 Hz, cisand trans-), 5.55 (t, 2JP-P ) 80.7 Hz, cis-); 5.43 (t, 2JP-P ) 79.4 Hz, trans-). 13C NMR: δ 150.61, 130.32, 126.61, 126.55, 126.28, 126.15, 122.26, 121.85, 121.75, 121.56. IR (neat): 3064, 1589, 1486, 1258, 1237, 1173, 1069, 960, 945, 878, 801, 767, 688, 638, 571, 530 cm-1. trans-/cis-2,4-Di(4-bromophenoxyl)-2,4,6,6-tetra(phenoxy)cyclotriphosphazene (trans-/cis-3). Sodium 4-bromophenoxide was prepared by reacting 4-bromophenol (2.851 g, 0.016 mol) in tetrahydrofuran (60 mL) and sodium hydride (0.69 g, 0.029 mol, 60% dispersion in mineral oil) at room temperature. Then trans-/ cis-2 (4.06 g, 0.0070 mol) dissolved in tetrahydrofuran (75 mL) was added dropwise to the solution of sodium 4-bromophenoxide. After the addition was complete, the reaction mixture was allowed to reflux and stir for 12 h under nitrogen. Then the THF was removed under reduced pressure and the residue was redissolved in chloroform. The organic layer was washed with water and dried with anhydrous magnesium sulfate. The drying agent was filtered off and the chloroform was removed. The crude product was chromatographed using chloroform/hexane as eluent to afford a

Hybrid Organic-Inorganic Polymers 9625 mixture of trans-3 and cis-3 as a white solid with a yield of 80%. Melting point: 112-115 °C. 1H NMR: δ 6.71-6.76 (t, 4H), 6.91-6.93 (m, 8H), 7.12-7.30 (m, 16H). 13C NMR: δ 150.68, 149.74, 132.44, 129.52, 125.03, 122.89, 120.98, 118.01. 31P NMR: δ 9.12-9.53 (t). IR (KBr): 3090, 3062, 3043, 3014, 1590, 1483, 1456, 1399, 1289, 1262, 1237, 1177, 1095, 1068, 1024, 1008, 951, 905, 890, 877, 834, 818, 811, 775, 729, 689, 637, 631, 617, 611, 587, 575, 539, 514 504 cm-1. EIMS (m/z): 850.3 (M+). Anal. Calcd for C36H28Br2N3O6P3: C, 50.79; H, 3.31; N, 4.94. Found: C, 50.42; H, 3.55; N, 5.12. trans-/cis-2,4-Di(4-(4-bromophenyl)phenoxy)-2,4,6,6-tetra(phenoxy)cyclotriphosphazene (trans-/cis-4). Yield: 78%. Trans/ cis-4 were prepared using a similar method to trans/cis-3. 4-(4bromophenoxyl)phenol (3.51 g, 0.014mol) reacted with trans/cis-2 (3.53 g, 0.0061mol) to give trans/cis-4 as a white solid with a yield of 78%. Melting point: 111-114 °C. 1H NMR: δ6.93-6.97 (t, 12H), 7.07-7.21 (m, 12H), 7.25-7.27 (d, 8H, J ) 8.2 Hz), 7.49-7.51 (d, 4H, J ) 8.2 Hz). 13C NMR: δ150.78, 150.49, 139.15, 136.64, 132.02, 129.47, 128.43, 127.80, 124.95, 124.84, 121.66, 121.52, 121.10. 31P NMR: δ9.26-9.58 (t). IR (KBr) 3061, 1590, 1513, 1486, 1455, 1387, 1270, 1207, 1175, 1161, 1074, 1024, 1004, 951, 945, 907, 887, 879, 854, 826, 816, 777, 770, 761, 709, 691, 573, 539, 501 cm-1. MS (ESI) (m/z): 1004.2 (M+). Anal. Calcd for C48H36Br2N3O6P3: C, 57.45; H, 3.62; N, 4.19. Found: C, 57.45; H, 3.37; N, 4.52. trans-/cis-2,4-Di(4-bromophenoxy)-2,4,6,6-tetrachlorocyclotriphosphazene (trans-/cis-6) and gem-2,2-di(4-bromophenoxy)-4,4,6,6-tetrachlorocyclotriphosphazene(geminal-6).trans-/ cis-6, and gem-6 were prepared according to the similar method of compound trans-/cis-2. Yield: 57%. The ratio of cis-6, trans-6, and gem-6 estimated from the integration of phosphorus in 31P NMR spectrum is 93.1%: 6.9% (31P NMR spectra of trans-6 and cis-6 are exactly same and nondistinguishable). 1H NMR: δ 7.52 (d, J ) 8.8 Hz), 7.47 (d, J ) 8.8 Hz), 7.16 (d, J ) 8.6 Hz), 7.06 (d, J ) 8.6 Hz). 13C NMR: δ 148.89, 133.37, 123.58, 123.32, 120.32, 120.23. 31P NMR: δ 25.43 (t, 2JP-P ) 65.2 Hz, cis- and trans-), δ24.62 (d, 2JP-P ) 64.6 Hz, gem-), 15.94 (d, 2JP-P ) 65.2 Hz, cisand trans-), -0.22 (t, 2JP-P ) 64.6 Hz, gem-). IR (KBr): 3093, 3072, 1582, 1482, 1399, 1292, 1175, 1068, 1012, 966, 871, 827, 871, 827, 760, 741, 665, 609, 538, 514 cm-1. EIMS (m/z): 620.8 (M+). Anal. Calcd for C12H8Br2Cl4N3O2P3: C, 23.22; H, 1.30. Found: C, 23.01; H, 1.51. trans-2,4-di(4-bromophenoxy)-2,4,6,6-tetra(ethoxy)cyclotriphosphazene (trans-7). trans-7 and cis-7 were prepared according to the similar method of compound trans/cis-2. Yield: 30%. Mp: 47-50 °C. 1H NMR: δ 7.41 (d, 4H, J ) 8.6 Hz), 7.14 (d, 4H, J ) 8.6 Hz), 3.93 (m, 4H), 3.73 (m, 4H), 1.24 (t, 6H, J ) 7.0 Hz), 1.19 (t, 6H, J ) 7.0 Hz). 13C NMR: δ 150.61, 132.60, 123.58, 118.00, 63.11, 62.31 (d, 2JC-P ) 5.3 Hz), 16.31, 16.25. 31P NMR: δ P(OC2H5)2, 17.17 (t), 16.66 (m), 16.20 (t); P(OC2H5)(OC6H4Br), 14.56 (s), 14.06 (s). IR (KBr): 3092, 2979, 2932, 2903, 1583, 1483, 1394, 1249, 1185, 1044, 944, 891, 836, 792, 752, 701, 610, 557 cm-1. EIMS (m/z): 659.1 (M+). Anal. Calcd for C20H28Br2Cl4N3O6P3: C, 36.44; H, 4.28. Found: C, 36.71; H, 4.34. cis-2,4-di(4-bromophenoxy)-2,4,6,6-tetra(ethoxy)cyclotriphosphazene (cis-7). Yield: 35%. Mp: 30-32 °C. 1H NMR: δ 7.33 (d, 4H, J ) 8.7 Hz), 6.96 (d, 4H, J ) 8.7 Hz), 4.13 (m, 4H), 3.52 (m, 4H), 1.35 (t, 6H, J ) 6.0 Hz), 1.32 (t, 3H, J ) 5.8 Hz), 1.08 (t, 3H, J ) 7.0 Hz). 13C NMR: δ 150.51, 132.58, 123.24, 117.84, 63.14, 62.39 (d, 2JC-P ) 5.3 Hz), 62.24 (d, 2JC-P ) 5.3 Hz), 16.36, 16.32, 16.22, 16.14. 31P NMR: δ P(OC2H5)2, 17.23 (t), 16.73 (m), 16.22 (t); P(OC2H5)(OC6H4Br), 14.52 (s), 14.02 (s). IR (KBr): 3088, 3064, 2977, 2934, 2905, 1587, 1484, 1250, 1217, 1187, 1161, 1037, 966, 948, 933, 902, 885, 834, 791, 755, 743, 612, 520 cm-1. EIMS (m/z): 659.0 (M+). Anal. Calcd for C20H28Br2Cl4N3O6P3: C, 36.44; H, 4.28. Found: C, 36.53; H, 4.47. trans-/cis-2,4-Di(4-(4-bromophenyl)phenoxy)-2,4,6,6-tetrachlorocyclotriphosphazene (trans-/cis-8), and gem-2,2-Di(4-(4bromophenyl)phenoxy)-4,4,6,6-tetrachlorocyclotriphosphazene (gem-8). trans-8, cis-8, and gem-8 were prepared according to the similar method of compound trans/cis-2. Yield: 61%. The

9626 Xu et al. ratio of cis-8, trans-8 and gem-8 estimated from the integration of phosphorus in 31P NMR spectrum is 56.6%:43.0%:0.4%. Mp: 120-124 °C. 1H NMR: δ 7.57 (d, J ) 8.4 Hz), 7.53 (d, J ) 8.4 Hz), 7.48 (d, J ) 8.6 Hz), 7.42 (d, J ) 8.4 Hz), 7.35 (d, J ) 8.4 Hz), 7.34 (d, J ) 8.3 Hz), 7.24 (d, J ) 8.4 Hz). 13C NMR: δ149.63, 139.19, 138.98, 138.89, 138.70, 132.45, 129.08, 128.95, 128.79, 128.68, 122.46, 122.41, 122.25, 122.04. 31P NMR: δ 25.50 (t, 2JP-P ) 65.5 Hz, cis-), 25.30 (t, 2JP-P ) 64.4 Hz, trans-), 24.48 (d, 2JP-P ) 63.8 Hz, gem-), 16.06 (d, 2JP-P ) 65.5 Hz, cis-), 15.89 (d, 2JP-P ) 64.4 Hz, trans-), -0.14 (t, 2JP-P ) 63.8 Hz, gem-). IR (KBr): 3061, 3047, 1602, 1587, 1510, 1479, 1245, 1224, 1180, 1159, 985, 964, 872, 817, 755, 661, 605, 588, 543, 509 cm-1. EIMS (m/z): 772.9 (M+). Anal. Calcd for C24H16Br2Cl4N3O2P3: C, 37.29; H, 2.09. Found: C, 37.12; H, 2.23. trans-2,4-di(4-(4-bromophenyl)phenoxy)-2,4,6,6-tetra(ethoxy)cyclotriphosphazene (trans-9). trans-9 and cis-9 were prepared according to the similar method of compound trans/cis-2. Yield: 35%. Mp: 130-132 °C. 1H NMR: δ 7.55 (d, 4H, J ) 8.5 Hz), 7.48 (d, 4H, J ) 8.6 Hz), 7.39 (d, 4H, J ) 8.5 Hz), 7.33 (d, 4H, J ) 8.6 Hz), 3.98 (m, 4H), 3.75 (m, 4H), 1.25 (t, 6H, J ) 7.1 Hz), 1.17 (t, 6H, J ) 7.1 Hz). 13C NMR: δ 151.36, 139.74, 136.99, 132.33, 128.90, 128.14, 122.22, 121.90, 63.02, 62.23 (d, 2JC-P ) 5.2 Hz), 16.40, 16.32.31P NMR: δ P(OC2H5)2, 17.36 (t), 16.86 (m), 16.35 (t); P(OC2H5)(OC6H4C6H4Br), 14.62 (s), 14.11 (s). IR (KBr): 2982, 2903, 1603, 1513, 1480, 1390, 1254, 1214, 1195, 1164, 1074, 1038, 951, 935, 891, 817, 795, 764, 710, 554 cm-1. EIMS (m/z): 811.1 (M+). Anal. Calcd for C32H36Br2N3O6P3: C, 47.37; H, 4.47. Found: C, 47.56; H, 4.56. cis-2,4-di(4-(4-bromophenyl)phenoxy)-2,4,6,6-tetra(ethoxy)-cyclotriphosphazene (cis-9). Yield: 40%. Mp: 139-141 °C. 1H NMR: δ 7.48 (d, 4H, J ) 8.4 Hz), 7.34 (d, 4H, J ) 8.6 Hz), 7.27 (d, 4H, J ) 8.5 Hz), 7.15 (d, 4H, J ) 8.5 Hz), 4.18 (m, 4H), 4.02 (m, 2H), 3.49 (m, 2H), 1.38 (t, 6H, J ) 7.1 Hz), 1.33 (t, 3H, J ) 7.1 Hz), 1.01 (t, 3H, J ) 7.0 Hz). 13C NMR: δ 150.86, 139.14, 136.23, 131.93, 128.37, 127.63, 121.61, 121.48, 62.73, 61.98 (d, 2JC-P ) 5.2 Hz), 61.74 (d, 2JC-P ) 5.0 Hz), 16.07, 15.93, 15.85. 31P NMR: δ P(OC2H5)2, 17.49 (t), 16.99 (m), 16.48 (t); P(OC2H5)(OC6H4C6H4Br), 14.70 (s), 14.19 (s). IR (KBr): 2931, 1604, 1514, 1481, 1390, 1252, 1215, 1187, 1075, 1039, 947, 896, 817, 761, 546 cm-1. EIMS (m/z): 811.2 (M+). Anal. Calcd for C32H36Br2N3O6P3: C, 47.37; H, 4.47. Found: C, 47.44; H, 4.67. General Synthetic Procedures for Polymers P1-12. A mixture containing monomer 3, (or 4, or 7, or 9), Pd(PPh3)4, 9,9dialkylflourene-2,7-bis(trimethyleneborate) (5) in degassed toluene and 2 M aqueous K2CO3 solution and a catalytic amount of Aliquat 336 was vigorously stirred at 120 °C for 3 days under nitrogen. The mixture was then cooled to room temperature and poured into a mixture of water and hydrazine. The mixture suspension was filtered and residue was extracted with CHCl3. The combined organic extract was washed with dilute HCl and water. The organic layer was dried and was evaporated to dryness. The residue was then redissolved in a small amount of toluene. The toluene solution prepared above was slowly poured into cold methanol again. This reprecipitation process was repeated for three times, followed by extraction in a Soxhelt apparatus to afford the desired polymers. P1. Yield: 73%. 1H NMR: δ 7.41-7.82 (m, 10H), 6.85-7.21 (m, 24H), 2.04 (br, s, 4H), 1.07 (br, s, 12H), 0.72 (m, 10H). 31P NMR: δ 8.9-9.9 (m). IR (KBr): 3065, 3042, 2927, 2856, 1592, 1509, 1489, 1465, 1270, 1202, 1160, 1070, 1024, 1007, 949, 903, 880, 817, 766, 729, 687 cm-1. Anal. Calcd for (C61H60N3O6P3)n: 71.54; H, 5.91; N, 4.10. Found: C, 71.45; H, 6.26; N, 4.10. P2. Yield: 62%. 1H NMR: δ 7.38-7.59 (m, 10H), 6.90-7.24 (m, 24H), 2.08 (br, s, 4H), 0.85 (br, s, 12H), 0.53-0.63 (br d, 10H). 31P NMR δ 8.9-9.9 (m). IR (KBr): 3065, 3042, 2955, 2922, 2855, 1592, 1510, 1490, 1465, 1269, 1202, 1179, 1069, 1024, 1007, 950, 903, 880, 848, 817, 767, 730, 687 cm-1. Anal. Calcd for (C65H68N3O6P3)n: C, 72.28; H, 6.35; N, 3.89. Found: C, 71.98; H, 6.39; N, 3.94. P3. Yield: 69%. 1H NMR: δ 7.40-7.82 (m, 10H), 6.84-7.21 (m, 24H), 2.04 (br, s, 4H), 1.08 (br, s, 20H), 0.76 (m, 10H). 31P NMR: δ 9.1-10.0 (m). IR (KBr): 3065, 3042, 2926, 2853, 1592,

Macromolecules, Vol. 41, No. 24, 2008 1509, 1490, 1465, 1269, 1203, 1178, 1160, 1069, 1007, 1016, 1025, 950, 903, 880, 848, 817, 766, 729, 687 cm-1. Anal. Calcd for (C65H68N3O6P3)n: C, 72.28; H, 6.35; N, 3.89. Found: C, 71.10; H, 6.82; N, 3.84. P4. Yield: 67%. 1H NMR: δ 7.40-7.82 (m, 10H), 6.84-7.21 (m, 24H), 2.06 (br, s, 4H), 1.00-1.29 (br, s, 36H), 0.70-0.89 (m, 10H). 31P NMR: δ9.1-9.8 (m). IR (KBr): 3060, 3029, 2923, 2851, 1592, 1489, 1465, 1411, 1397, 1377, 1268, 1238, 1202, 1160, 1069, 1024, 1007, 950, 903, 881, 856, 767, 721, 688 cm-1. Anal. Calcd for (C73H84N3O6P3)n: C, 73.53; H, 7.10; N, 3.52. Found: C, 73.06; H, 7.25; N, 3.46. P5. Yield: 77%. 1H NMR: δ 7.41-7.82 (m, 20H), 6.93-7.22 (m, 22H), 2.06 (br, s, 4H), 1.06 (br, s, 12H), 0.74 (m, 10H). 31P NMR: δ 9.3-9.8 (t). IR (KBr): 3029, 2927, 2855, 1592, 1489, 1464, 1268, 1200, 1178, 1159, 1070, 1024, 1007, 949, 903, 880, 813, 766, 688 cm-1. Anal. Calcd for (C73H68N3O6P3)n: C, 73.54; H, 5.83; N, 3.57. Found: C, 74.46; H, 6.13; N, 3.72. P6. Yield: 72%. 1H NMR: δ 7.30-7.83 (m, 20H), 6.75-7.27 (m, 22H), 2.10 (br, s, 4H), 0.86 (br, s, 12H), 0.55-0.62 (br d, 10H). 31P NMR: δ 9.1-9.7 (m). IR (KBr): 3030, 2954, 2922, 2855, 1592, 1489, 1464, 1268, 1178, 1160, 1069, 1007, 1024, 950, 903, 880, 813, 766, 687 cm-1. Anal. Calcd for (C77H77N3O6P3)n: C, 75.04; H, 6.22; N, 3.41. Found: C, 74.42; H, 6.16; N, 3.40. P7. Yield: 71%. 1H NMR: δ 7.41-7.84 (t, 20H), 6.92-7.24 (m, 22H), 2.06 (br, s, 4H), 1.07 (br, s, 20H), 0.77 (m, 10H). 31P NMR: δ 9.2-9.8 (m). IR (KBr): 3031, 2928, 2850, 1592, 1496, 1464, 1270, 1159, 1069, 1024, 1007, 950, 881, 814, 766, 688 cm-1. Anal. Calcd for (C77H76N3O6P3)n: C, 75.04; H, 6.22; N, 3.41. Found: C, 74.74; H, 6.54; N, 3.35. P8. Yield: 70%. 1H NMR: δ 7.40-7.82 (m, 20H), 6.93-7.22 (m, 22H), 2.04 (br, s, 4H), 1.00-1.35 (br, m, 36H), 0.75-0.90 (br, m, 10H). 31P δ NMR: 9.2-9.8 (m). IR (KBr): 3066, 3042, 2924, 2852, 1593, 1509, 1490, 1466,, 1268, 1203, 1178, 1161, 1069, 1025, 1007, 950, 880, 818, 767, 688 cm-1. Anal. Calcd for (C85H92N3O6P3)n: C, 75.93; H, 6.90; N, 3.13. Found: C, 76.35; H, 6.87; N, 2.90. P9. Yield: 67%. 1H NMR: δ 7.37-7.77 (m, 14H), 4.01 (m, 4H), 3.79 (m, 4H), 2.04 (m, 4H)1.19-1.31 (m, 12H), 1.06 (m, 12H), 0.75 (m, 10H). 31PNMR: δ 16.44-17.45 (m, 1P, P(OC2H5)2), 14.38-14.88 (br. d, 2P, P(OC2H5)(Ar)). IR (KBr): 3028, 2926, 2856, 1602, 1511, 1463, 1392, 1253, 1191, 1166, 1040, 937, 889, 818, 746, 635, 548 cm-1. Anal. Calcd for C45H60N3O6P3: C 64.97; H 7.27. Found: C,65.49; H,7.10. P10. Yield: 69%. 1H NMR: δ 7.21-7.67 (m, 14H), 4.19 (m, 4H), 4.03 (m, 2H), 3.05(m, 2H), 2.00(br. S, 4H), 1.32-1.39 (m, 12H), 1.02 (m, 12H), 0.70 (m, 10H). 31P NMR: δ 16.23-17.49 (m, 1P, P(OC2H5)2), 14.16-14.66 (br. d, 2P, P(OC2H5)(Ar)). IR (KBr): 2927; 2855, 1604, 1509, 1466, 1251, 1214, 1190, 1164, 1036, 945, 934, 887, 816, 744, 539 cm-1. Anal. Calcd for C45H60N3O6P3: C 64.97; H 7.27. Found: C,65.50; H,7.42. P11. Yield: 72%. 1H NMR: δ 7.48-7.81 (m, 18H), 7.38-7.40 (m, 4H), 4.01 (m, 4H), 3.79 (m, 4H), 2.05 (br s 4H), 1.07-1.31 (m, 24H), 0.76 (t, 10H). 31P NMR: δ 17.60-16.50 (m, 1P, P(OC2H5)2), 14.36-14.87 (br. d, 2P, P(OC2H5)(Ar)). IR (KBr): 3028, 2926, 2855, 1603, 1498, 1463, 1391, 1253, 1189, 1163, 1038, 935, 888, 814, 757, 545 cm-1. Anal. Calcd for C57H68N3O6P3: C 69.57; H 6.96. Found: C 69.98; H, 7.25. P12. Yield: 75%. 1H NMR: δ 7.51-7.71 (m, 18H), 7.22-7.27 (m, 4H), 4.22 (m, 4H), 4.04 (m, 2H), 3.50 (m, 2H), 2.04 (br s 4H), 1.08-1.41 (m, 24H), 0.69 (t, 10H). 31P NMR: δ 17.55-16.55 (m, 1P, P(OC2H5)2), 14.25-14.75 (br. d, 2P, P(OC2H5)(Ar)). IR (KBr): 3030, 2926, 2855, 1604, 1499, 1463, 1391, 1252, 1209, 1188, 1163, 1087, 944, 889, 814, 756, 540 cm-1. Anal. Calcd for C57H68N3O6P3: C 69.57; H 6.96. Found: C 70.10; H, 6.85.

Results and Discussion Synthesis of Cyclotriphosphazene-Based Monomers and Polymers. The synthetic routes leading to polymers P1-8, P9-10, and P11-12 are outlined in Schemes 1, 2, and 3, respectively. Compound 2,4-dichloro-2,4,6,6-tetra(phenoxy)-


Hybrid Organic-Inorganic Polymers 9627

Scheme 1. Synthetic Route Leading to the Polymer P1-8a

a

Reaction conditions: (i) THF, room temperature; (ii) THF, reflux; (iii) Pd(PPh3)4, 2 M K2CO3, Aliquat 336, toluene, reflux at 105 °C, 3 days.

cyclotriphosphazene43 (trans/cis-2) was readily prepared from hexachlorocyclotriphosphazene (1) and sodium phenoxide in 60% yield. Reaction of compound trans-/cis-2 with sodium 4-bromophenoxide and sodium 4-(4-bromophenoxy)phenoxide afforded geometrical isomers trans-/cis-3 and trans-/cis-4 in 80% and 78% yields, respectively. Compound 6 (a mixture of cis-6, trans-6, and gem-6) was synthesized by reacting sodium 4-bromophenoxide with compound 1 in THF.44 Further reaction between a mixture of cis-6, trans-6, and gem-6, and excess sodium ethoxide in THF was carried out to afford pure compounds cis-7 and trans-7 in yields of 30 and 35%, respectively (Scheme 2). According to the similar synthetic strategy, pure compounds cis-9 and trans-9 was prepared in 40% and 35% yields, respectively (Scheme 3). The chemical structures of all intermediates 2-9 were unambiguously char-

acterized by FT-IR, 1H NMR, 13C NMR, 31P NMR spectroscopy, mass spectrometry, and elemental analysis. In general, the cis and trans-isomers of cyclotriphosphazene derivatives could be distinguished by using 31P and 1H NMR spectroscopy. In theory, 31P NMR spectra of 3 and 4 should exhibit an AB2 spin system, but the 31P chemical shift difference (∆δAB) between two phosphorus atom A and phosphorus atom B was very small, and thus only a triplet at δ 9-10, instead of a set of eight singlets, was observed. The chemical structures of compounds 2,43 6,44 and 8, including their geometrical isomers, were readily identified by 31P NMR spectroscopy on the basis of their chemical shifts and splitting patterns (AX2 spin system). However, the 31P NMR spectra were indistinguishable for the geometrical isomers of compounds 7 and 9. The 31P NMR and 1H NMR spectra of compounds cis-9 and

9628 Xu et al.

Macromolecules, Vol. 41, No. 24, 2008 Scheme 2. Synthetic Route Leading to the Polymer P9-10a

a

Reaction conditions: (i) THF, room temperature; (ii) THF, reflux; (iii) Pd(PPh3)4, 2 M K2CO3, Aliquat 336, toluene, reflux at 105 °C, 3 days. 31P

Figure 1. 31P NMR spectra of trans-9 (I and II) and cis-9 (III and IV) in CDCl3 at room temperature. Spectra I and III, 1H nuclei coupled; spectra II and IV, 1H nuclei decoupled.

trans-9 are shown in Figures 1 and 2, respectively. Compounds cis-9 and trans-9 exhibited a complicated 1H nuclei coupled

NMR spectra due to coupling between phosphorus and methylene protons (OCH2CH3), and they provided the same spectral profiles in which two sets of signals were attributed to two different environmental phosphorus atoms (P(OCH2CH3)2 and P(OCH2CH3)(OC6H4C6H4Br)) at δ 16.2-17.6 and 13.9-14.9, respectively, with an integration ratio of 1:2 (Figure 1 (I, III)). However, the complicated splitting patterns could be simplified to appear eight characteristic singlets, corresponding to a typical AB2 spin system, in their respective 1H nuclei decoupled 31P NMR spectra (Figure 1 (II, IV)). In the 1H NMR spectrum of cis-9 (Figure 2), only one OCH2CH3 group (c) is located in the shielding zone of the two biphenyl groups, resulting in upfield shifts of 0.53 and 0.32 ppm, for CH2 and CH3, respectively, relative to another geminal OCH2CH3 group (b) that is aligned below the cyclotriphosphazene ring. In contrast, for trans-9, two types of OCH2CH3 groups are located in the shielding zone of one biphenyl groups, and they show two sets of signals at δ 3.98 and 1.25, and δ 3.75 and 1.17, corresponding to P(OCH2CH3)(OC6H4C6H4Br) and P(OCH2CH3)2, respectively (Figure 2). Copolymers P1-12 were synthesized from cyclotriphosphazene-based monomers (3, 4, 7, or 9) and 9,9-dialkylflourene2,7-bis(trimethyleneborate) (5a-d) Via Pd(PPh3)4-catalyzed Suzuki cross-coupling reactions. All polymers were structurally characterized by spectroscopic methods and elemental analyses.



Scheme 3. Synthetic Route Leading to the Polymer P11-12a

a

Reaction conditions: (i) THF, reflux; (ii) THF, reflux; (iii) Pd(PPh3)4, 2 M K2CO3, Aliquat 336, toluene, reflux at 105 °C, 3days.

The 31P NMR spectra of the copolymers P1-8 and P9-12 clearly displayed signals in the range between δ 9-10 and 14.1-17.5, respectively, corresponding to characteristic phosphorus signals of cyclotriphosphazenes. This suggested that the cyclotriphosphazene rings were integrated into the polymer backbone, and they remained intact during the polymerization process. As an example, 31P and 1H NMR spectra of polymer 5 are illustrated in Figure 3. Molecular weights were determined by gel permeation chromatography (GPC) against a polystyrene standard, and thermal stabilities in air and inert atmosphere are summarized in Table 1. Polymers P5-8 and P11-12 have higher molecular weights (Mn) than the corresponding polymers P1-4 and P9-10. All polymers were found to be completely soluble in common organic solvents such as chloroform, THF, toluene, and xylenes at ambient temperature. Thermal and X-ray Diffraction Analysis of Polymers. Thermogravimetric analysis (TGA) was used to evaluate the thermal stability of the polymers P1-12. For example, the TGA thermograms for polymers P1-4 and P5-8 in nitrogen are shown in Figure 4. Polymers P1-8 exhibit very good thermal stability with decomposition temperatures (Td) in the range of 419-446 °C in nitrogen (Table 1). The high Td’s for these

polymers are primarily due to the presence of inorganic cyclotriphosphazene cores, which enhance the thermal stability of the polymers. The glass transition temperatures (Tg) of all polymers were also clearly detected by differential scanning calorimetry (DSC). For these two series, P1-4 and P5-8, Tg’s progressively decreased with an increase in the length of dialkyl chains at 9-position of the fluorene moiety (Figure 5). Tg’s dropped from 90 to 39 °C when the alkyl chain was changed from hexyl to dodecyl in the P5-8 polymer series. P5-8 exhibited relatively higher Tg values than the corresponding P1-4 polymers with the same alkyl groups. This is because the P5-8 have longer rigid repeating units than those of the P1-4 polymer series. However, polymers P9-12 displayed inferior thermal stabilities when compared with P1-8 by nearly 100 °C. It is also worthy noting that polymers P10 and P12, derived from trans-monomers, have better thermal stabilities and higher Tg’s than the corresponding polymers P9 and P11 derived from cis-monomers. The packing of polymers P1-12 in the solid state was found to be amorphous by wide-angle X-ray diffraction experiments, with two broad halos observed at 2θ ) 9-12° (∼7.7-9.6 Å) and 19-22° (∼4.1-4.5 Å) in the diffraction diagrams, as shown

9630 Xu et al.

Figure 2. 1H NMR spectra of cis-9 (bottom) and trans-9 (top) with undeuterated CHCl3 in solvent.


31

P nuclei coupled in CDCl3 at room temperature. (/) Residual water and Table 1. Synthetic Yields, Molecular Weights, Mw/Mn and

Thermal Data of Polymers P1-P8.a

polymers yield (%)

Mw

Mn

Mw/Mn Tg Td (in N2) Td (in air)

P1 73 17 300 9900 1.76 53 446 435 P2 62 18 600 9600 1.94 49 423 403 P3 69 11 900 7300 1.63 43 447 412 P4 67 32 600 14 500 2.15 16 436 382 P5 77 24 100 14 600 1.65 90 449 435 P6 72 26 500 13 500 1.96 71 419 403 P7 71 38 000 16 400 2.31 63 436 412 P8 70 25 100 11 700 2.24 39 436 419 P9 67 17 900 9300 1.92 45 292 289 P10 69 15 700 8500 1.85 59 329 326 P11 72 16 000 10 400 1.54 62 337 337 P12 75 33 200 14 100 2.35 83 355 349 a Mw: the weight-average molecular weight. Mn: the number-average molecular weight. Tg: glass transition temperature. Td: the decomposition temperature at which 5% weight loss occurs.

Figure 3. 31P NMR (top) and 1H NMR (bottom) spectra of P5. (/) Residual water and undeuterated CHCl3.

in Figure 6. The broad diffraction peaks with a d-spacing of 4.1-4.5 Å are ascribed to intermolecular alkyl-alkyl and intramolecular aryl-aryl interactions. The diffraction peaks centered at 2θ ) 9-12°, with a d-spacing of 7.7-9.6 Å, are assigned to the average polymer interchain distance. The relative longer interchain distances indicate that the introduction of cyclotriphosphazene units suppress the aggregation of polymer chains. From Table 2, it is clear that the length of alkyl chains at the 9-postiton of the fluorene groups in P1-8 has little effect on the magnitude of the interchain distances, suggesting the presence of interdigitated structures. Polymers P5-8 and P11-12, with a longer and more rigid backbone than P1-4 and P9-10, have slightly longer interchain distances by ∼0.5 Å. Furthermore, the stereochemistry of the monomers was found to have a negligible effect on the average interchain distances.

In comparison with polymers P9-12 with ethoxy substituents in the cyclotriphosphazene ring, polymers P1-8 have the longer average interchain distances by 1 Å because the relatively larger phenoxy groups in P1-8 repel the interchain contacts such that they display slightly longer interchain distances. Optical Properties of Polymers. The UV-vis and fluorescence spectra of polymers P1-12 were determined and are summarized in Table 3. Transparent and uniform films of polymers were prepared readily on quartz plates by spin-casting their THF solutions at room temperature. The UV-vis and photoluminescence spectra in both the solution and the film state are shown in Figures 7, 8, and 9. Polymers P1-12 displayed a small red-shift of the 0-0 absorption bands in film compared to that in solution (∆λmax ) 1-4 nm; Table 2), suggesting that there is little change in molecular conformation from the solution to the film state. For polymers P1-4 and P9-10, the absorption maxima were around 330 nm, while the absorption maxima for polymers P5-8 and P11-12 were red-shifted by 10 nm (341-345 nm), because P5-8 and P11-12 have longer conjugation lengths than P1-4 and P9-10. However, all polymers in film state experienced some emission red-shifts



Figure 4. TGA curves of P1-4 (a) and P5-8 (b) in N2.

Figure 5. DSC curves of P1-P4 (a) and P5-P8 (b) in N2. Table 2. X-ray Diffraction Data polymers

diffraction angle 2θ (deg)

d-spacing (Å)

P1 P2 P3 P4 P5 P6 P7 P8 P9 P10 P11 P12

9.95, 19.93 9.88, 20.12 9.92, 20.41 9.94, 20.24 9.35, 19.55 9.56, 20.18 9.40, 20.21 9.25, 20.83 11.40, 22.07 11.34, 21.60 10.61, 21.50 10.35, 21.55

8.89, 4.45 8.95, 4.41 8.92, 4.35 8.90, 4.39 9.46, 4.54 9.25, 4.40 9.41, 4.39 9.56, 4.26 7.76, 4.03 7.80, 4.11 8.34, 4.13 8.55, 4.12

relative to those in solution. Polymers P1-4 and P9-10, with short conjugation lengths, underwent more significant emission red-shifts (29-35nm) from the solution to the film state as compared to polymers P5-8 (17-21nm). Unlike polymers P9-10, polymers P11-12 experienced significantly lesspronounced emission red-shifts from solution to the film state (3-10 nm). The types of substituents in the cyclotriphosphazene ring have diverse effects on the absorption and emission maxima. For example, polymers P9-10 with four ethoxy substituents exhibited similar absorption and emission characteristics to

P1-4 with four phenoxy substituents. P9-10 displayed two emission maxima at 363-365 and 382-384 nm, which is very close to those of P1-4, with λmax ) 363-364 nm, 382-385 nm. In contrast, P11-12 (λmax ) 397-399, 410-414 nm) showed less emission maxima than P5-P8 (λmax ) 406-408, 426-428 nm) in the film state (Table 3). The stereochemistry of the polymers plays an additional role in affecting the optical properties. For instance, P10 and P12, derived from cis-monomers, displayed slightly lower absorption maxima but larger emission maxima both in solution and in the film state relative to P9 and P11, which were prepared from trans-monomers. All polymers displayed narrow full width at half-maximum (fwhm) of the emission spectra in the range of 45-54 nm, suggesting that these polymers have good color purities. The photoluminescence efficiencies (φPL) of the polymers in THF were measured by using quinine sulfate as a standard. The φPL of polymers P1-8 is between 0.55 and 0.69, and it is less than the φPL values of polymers P9-12 (0.73-0.88). The change in φPL values is likely related to the special alignment of substituents in the cyclotriphosphazene ring. Single crystal X-ray analysis of hexa(aryloxy)cyclotriphosphazene shows that the six aryloxy groups lie almost perpendicularly above and below the nearly planar cyclotriphosphazene core,45-47 and this structural

9632 Xu et al.


Figure 6. Wide angle X-ray diffraction profiles of the polymers (a) P1-4 and (b) P5-8. Intensity is offset for clarity. Table 3. Optical Properties of Polymers P1-P8 UV (nm) polymers

λmax (in THF)

λmax (in film)

fluorescence (nm) ∆λmaxb

λmax (in THF)

λmax (in film)

fwhm (in film)

φPLa (in THF)

∆λmaxb

P1 330 332 2 364, 382, 402 (sh)c 394, 411 52 0.61 30, 29 P2 330 332 2 363, 382, 402 (sh)c 394, 410 52 0.60 31, 28 c P3 331 333 2 364, 385, 407 (sh) 399, 414 52 0.69 35, 29 P4 331 333 2 364, 384, 407 (sh)c 404 50 0.57 n. a.d P5 341 343 2 389, 408 412 54 0.55 n. a.d P6 341 342 1 389, 406 406, 426 (sh) 48 0.63 17, 20 P7 342 345 3 390, 408 410, 428 (sh) 48 0.59 20, 20 P8 341 345 4 380, 407 408, 428 (sh) 45 0.65 20, 21 P9 330 333 3 363, 382 392, 411 48 0.81 29, 29 P10 329 330 1 365, 384 392, 412 45 0.81 27, 28 P11 343 344 1 387, 407 397, 410 52 0.88 10, 3 P12 340 341 1 390, 410 399, 414 58 0.73 9, 4 a The PL efficiency (φPL) of the polymers in THF solutions was measured by using quinine sulfate (ca. 1 × 10-5 M solution in 0.1 M H2SO4, assuming φPL of 0.55) as a standard. Absorption maxima of P1-12 were taken as λex (excitation wavelength) for P1-P8. b ∆λmax ) λmax (in film)λmax(in solution). c sh ) shoulder peak. d n.a. ) not applicable.

Figure 7. Photoluminescence spectra of P1-2 and P5-6 (a) and P3-4 and P7-8 (b) in THF.

feature makes it possible for the three aryloxy groups to be located on the same side of the (PN)3 plane, able to align through intramolecular π-π interactions. In fact, the downfield chemical shift of the aromatic protons in cis-9 relative to corresponding aromatic protons in trans-9 suggests the presence of such an

aryl-aryl interaction. Molecular modeling of compound trans-3 by Chem3D 8.0 shows that the biphenyl group cofacially interacts with one adjacent phenoxy group linked to the neighboring phosphorus atom. The centroid distance of the two aromatic groups is 3.48 Å, very close to the normal π-π



Figure 8. Photoluminescence spectra of P1-2 and P5-6 (a) and P3-4 and P7-8 (b) in film state.

Figure 9. Photoluminescence spectra of P9-12 (a) in THF and (b) in the film state.

stacking contact. This type of noncovalent π-π interaction has been demonstrated to be able to result in the emission spectral shifts accompanied by changes in φPL.48,49 Therefore, the abovementioned emission change ∆λmax (17-35 nm) from the solution state to the film state for polymers P5-8 is also likely to be due to these intramolecular π-π interactions. The spectral shifts and the variations in φPL values can be also explained by an electron transfer mechanism. The possibility of intramolecular electron transfer can be roughly estimated by the Rehm-Weller equation, which approximates the free energy change (∆G#) between a donor and an acceptor.50,51 However, in our studied polymers system, the four phenoxy groups serve as acceptors and interact with one rigid backbone repeat unit (donor), making it impracticable to reliably estimate the value of ∆G# using this equation. Thermal Effect on Optical Stability. In order to avoid the possible oxidation of C-9 in the fluorene unit, the emission spectra of polymers P1-8, using P3 and P5 as examples, were investigated at 200 °C under vacuum and ambient light. As evident by TGA experiments, polymers P1-8 are stable and do not experience any degradation at this temperature. Therefore, any green emission observed under vacuum conditions should

be attributed to a reordering of the polymer chain, and the subsequent aggregate formation, as the absence of oxygen rules out the oxidation of the potentially vulnerable C-9 of the fluorene unit to produce fluorenone moieties and, therefore, eliminates the possibility of keto defect-induced green emission. The emission spectra of P3 and P5 film after thermal annealing for 5 and 20 h are illustrated in Figure 10. Upon heating for 5 h at 200 °C, polymer P3 produced nearly the exact same spectral profiles as its pristine film, and no excimer green emission was observed. Prolonging the thermal annealing did not result in additional green emission. Similar optical behaviors of polymer P5 were observed under the same thermal treatment conditions, suggesting that the three-dimensional structure of the polymers prevented aggregation of the polymer chains and, hence, that it nearly completely suppressed formation of aggregation-induced excimer emission. In contrast, poly(9,9-dihexylfluorene) showed a strong green emission band centered at 520 nm in its emission spectrum upon annealing in nitrogen for 3.5 h.19 Theses observed results are consistent with the fact that amorphous structure of our polymers, which is unfavorable toward aggregate formation, as evidenced by XRD experiments. The phenoxy substituents in the cyclotriphosphazene cores push away the

9634 Xu et al.


Figure 10. Effect of thermal aging on PL spectra of (a) P3 and (b) P5 under ambient light and vacuum at 200 °C.

Figure 11. Effect of thermal aging on PL spectra of (a) P1 and (b) P7 under ambient light and in air at 200 °C.

Figure 12. I-V-L characteristics of the PLED device.

backbone chains and subsequently make the interchain distance larger, preventing the formation of excimers.

The emission spectra of P1-8 in air at 200 °C for 5 h were also examined to provide insight into how the thermal stability



Table 4. Electroluminescence Data of PLED Devicesa

polymers

Von (V)b

maximum brightness (cd/m2) at 15 V

maximum current density (mA/cm2)

maximum current efficiency (Cd/A)

luminance efficiency (lm/W)

P5 7 240 656 0.04 0.08 P7 5 410 870 0.05 0.12 a Devices structure: ITO/PDOT/polymer/Ca/Ag. b Von is the turn-on voltage of current.

and oxidation-resistant ability of polymers influence the formation of green emission induced by keto defects. The emission spectra of P1 and P7 film after thermal annealing together with their individual pristine films are shown in Figure 11. P1 exhibited a very weak featureless broadband the 500-600 nm region. In contrast, P7 displayed a relatively stronger green emission band at 525 nm in air than P1, possibly resulting from monomeric fluorenone charge-transfer π-π* transitions and energy transfer from the fluorene to the fluorenone moiety. This is significantly different than the optical behavior of poly(9,9dihexylfluorene) in air at the same temperature for 3-4 h, in which a strong green emission was observed.19,20,22 Nonetheless, the original emission bands at 394 and 411 nm for P1 and 410 and 428 nm for P7 are still dominant in both emission spectra of P1 and P7. Other polymers derived from monomers 3 and 4 also exhibited similar behaviors under annealing at the same temperatures. These results suggested that introduction of inorganic cyclotriphosphazene cores effectively retarded the oxidation of C-9 in the fluorene unit, thereby reducing keto defect-induced green emission. Electroluminescence Properties of Polymers. Polymers P5 and P7 were fabricated into four-layer PLED devices, with a configuration of ITO/PDOT:PSS/P5 or P7/Ca/Ag, using standard procedures of spin-coating. Under a biased voltage, the two PLED devices showed blue emissions with EL λmax of 416 nm. The current-voltage and luminescence-voltage curves of P5 and P7 PLED devices are shown in Figure 12, and the electroluminescence data of the PLED devices are summarized in Table 4. The turn-on voltages of these PLED devices were 5 and 7 V, and their maximum current efficiencies and maximum luminance were 0.04 and 0.05 cd A-1 and 240 and 410 cd m-2 (at 15 V), respectively. The current density increased with increasing forward bias voltage, indicating typical rectifying characteristics. The relatively higher brightness in P7 than P5 may be due to the relatively higher quantum efficiency of P7 than P5. The PLED brightness and efficiency are not high for these two materials, but we believe that they could be improved by optimizing the device architecture. These preliminary results seem to suggest that these polymers have the potential for application as PLED materials. Conclusions Cyclotriphosphazene-based polymers were synthesized from suitable cyclotriphosphazene precursors through Suzuki cross coupling reactions. All polymers provided blue light emission in high quantum yields. Incorporation of hybrid organic-inorganic cyclotriphosphazene cores in those polymers significantly enhanced the overall thermal stability of the polymers. The polymers are amorphous solids, as evidenced by two broad diffraction peaks at 7.7-9.6 Å and 4.1-4.5 Å. The introduction of the three-dimensional organic-inorganic hybrid architectures of the cyclotriphosphazene units prevented polymer aggregation and consequently diminished the formation of green excimer emission. The presence of the inorganic component ((PN)3) also significantly reduced the green emission induced by polymer oxidation. In addition, the particular arrangement of substituents in the cyclotriphosphazene ring allowed the aryl groups to stack

through intramolecular aromatic π-π interactions, leading to a spectral red-shift. The suppression of polymer aggregation and enhancement ofthermalstabilitybyintroducingthree-dimensionalorganic-inorganic hybrid architecture in the present polymer system allow for the prediction of a substantial reduction in both aggregation-induced excimer emission and keto-defect-induced emission. In turn, it offers an alternative way in which to design and synthesize nonaggregating thermally stable luminescent polymers with good color purity. Studies of the other types of thermally stable luminescent polymers and oligomers prepared starting from cyclotriphosphazene are under investigation in our laboratory. Acknowledgment. The authors would like to thank the Agency for Science, Technology and Research (A*STAR) for financial support. Note Added After ASAP Publication. This article was published ASAP on October 23, 2008. Two values in Table 4 have been changed. These same values have also been changed in the paragraph “Electroluminescence Properties of Polymers.” The revised manuscript was published on October 31, 2008. In the version published October 31, 2008, Table 4 has been modified. The corrected version was published on November 18, 2008. References and Notes (1) (a) Donat-Bouillud, A.; Levesque, I.; Tao, Y.; D’Iorio, M. Chem. Mater. 2000, 12, 1931–1936. (b) Bernius, M. T.; Inbasekaran, M.; O’Brien, J.; Wu, W. AdV. Mater. 2000, 12, 1737–1750. (c) Leclerc, M. J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 2867–2873. (2) (a) Zhou, X.-H.; Niu, Y.-H.; Huang, F.; Liu, M. S.; Jen, A. K.-Y. Macromolecules 2007, 40, 3015–3020. (b) Scherf, U.; List, E. J. W. AdV. Mater. 2002, 14, 477–487. (c) Rees, I. D.; Robinson, K. L.; Holmes, A. B.; Towns, C. R.; O’Dell, R. MRS Bull. 2002, 27, 451– 455. (3) (a) Allard, S.; Forster, M.; Souharce, B.; Thiem, H.; Scherf, U. Angew. Chem., Int. Ed. 2008, 47, 4070–4098. (b) Yasuda, T.; Fujita, K.; Tsutsui, T.; Geng, Y.; Culligan, S. W.; Chen, S. H. Chem. Mater. 2005, 17, 264–268. (c) Bolognesi, A.; Dicarlo, A.; Lugli, P. Appl. Phys. Lett. 2002, 81, 4646–4648. (d) Zhu, Y.; Babel, A.; Jenekhe, S. A. Macromolecules 2005, 38, 7983–7991. (4) (a) Nehls, B. S.; Asawapirom, U.; Fuldner, S.; Preis, E.; Farrell, T.; Scherf, U. AdV. Func. Mater. 2004, 14, 352–356. (b) Meng, H.; Bao, Z.; Lovinger, A. J.; Wang, B.; Mujsce, A. M. J. Am. Chem. Soc. 2001, 123, 9214–9215. (c) Meng, H.; Zheng, J.; Lovinger, A. J.; Wang, B.C.; van Patten, P. G.; Bao, Z. Chem. Mater. 2003, 15, 1778–1787. (5) (a) Lemmer, U.; Heun, S.; Mahrt, R. F.; Scherf, U.; Hopmeier, M.; Siegner, U.; Goebel, E. O.; Muellen, K.; Baessler, H. Chem. Phys. Lett. 1995, 240, 373–378. (b) Neher, D. Macromol. Rapid Commun. 2001, 22, 1365–1385. (6) (a) Leclere, P.; Hennebicq, E.; Calderone, A.; Brocorens, P.; Grimsdale, A. C.; Mullen, K.; Bredas, J. L.; Lazzaroni, R. Prog. Polym. Sci. 2003, 28, 55–81. (b) Herz, L. M.; Phillips, R. T. Phys. ReV. B 2000, 61, 13691–13679. (7) Lu, S.; Fan, Q.-L.; Chua, S.-J.; Huang, W. Macromolecules 2003, 36, 304–310. (8) Sims, M.; Bradley, D. D. C.; Ariu, M.; Koeberg, M.; Asimakis, A.; Grell, M.; Lidzey, D. G. AdV. Funct. Mater. 2004, 14, 765–781. (9) Chochos, C. L.; Tsolakis, P. K.; Gregoriou, V. G.; Kallitsis, J. K. Macromolecules 2004, 37, 2502–2510. (10) Surin, M.; Hennebicq, E.; Ego, C.; Marsitzky, D.; Grimsdale, A. C; Muellen, K.; Bredas, J.-L.; Lazzaroni, R.; Leclere, P. Chem. Mater. 2004, 16, 994–1001. (11) List, E. J. W.; Guntner, R.; Scandiucci de Freitas, P.; Scherf, U. AdV. Mater. 2002, 14, 374–378. (12) Lupton, J. M.; Craig, M. R.; Meijer, E. W. Appl. Phys. Lett. 2002, 80, 4489–4491. (13) Gaal, M.; List, E. J. W.; Scherf, U. Macromolecules 2003, 36, 4236– 4237. (14) Romaner, L.; Pogantsch, A.; de Freitas, P. S.; Scherf, U.; Gaal, M.; Zojer, E.; List, E. J. W. AdV. Funct. Mater. 2003, 13, 597–601. (15) Wu, Y.-S.; Li, J.; Ai, X.-C.; Fu, L.-M.; Zhang, J.-P.; Fu, Y.-Q.; Zhou, J.-J.; Li, L.; Bo, Z.-S. J. Phys. Chem. A 2007, 111, 11473–11479. (16) Setayesh, S.; Grimsdale, A. C.; Weil, T.; Enkelmann, V.; Mullen, K.; Meghdadi, F.; List, E. J. W.; Leising, G. J. Am. Chem. Soc. 2001, 123, 946–953.

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